A process mixer is one of the least understood pieces of rotating equipment in a process plant. There are literally hundreds of them from small portable mixers to large main reactor mixers. Because it is similar to a gear box, and lumped in with other rotating equipment, its performance is seldom questioned. However, if an L10 life calculation were performed you would realize the equipment should last 25 years. However, because other equipment last 5 years, a 5-year life is acceptable. There may be problems and you don’t even know it.
A mixer drive is not the same as a gear box. A gear box is stationary and only transmits torque. A mixer drive not only transmits torque, it has to carry the overhung loads. At rest there is an axial force due to the weight of the shaft and impeller. While in motion the action of the impeller will create additional axial loads, bending moments and torsion on the shaft. All of these loads have to be supported by the mixer drive. As a consequence a mixer drive should be several times larger than a gear box for a similar amount of torque. Not sizing a mixer properly leads to drive failure and lost impeller blades.
Detecting problems with a mixer can be somewhat challenging. Often times the only monitoring point is the motor. As long as the motor is turning, it is assumed the mixer is turning and the process is being agitated. It is common problem for the motor to be rotating, but the mixer be stationary. It is even more common for the impeller to be missing blades, due to flutter in the process. There has not been a good way to detect these issues until the process is producing product that does not meet specification. Granted, there are other indicators of a problem, but this tends to be the most common way.
Using a differential pressure (DP) level transmitter with advanced diagnostics is a great strategy to detect problems with you process mixer. Not only is it measuring the DP in the vessel, it is also monitoring the mean and standard deviation of the pressure signal. This is similar to the speedometer on the car. The rotation of the tires provide the speed, but the tires feel every part of the road. Should the car leave pavement and proceed on gravel, the speed is the same, but the tires feel something different. The pressure transmitter sensors realize the same process signals.
When a mixer is rotating in a process there are pressure pulses that are created for each impeller blade. If there mixer wear to stop, the pressure pulses would be much different. The transmitter with advanced diagnostics would detect this change and report an error. The same scenario would occur if the impeller lost a blade. A standard four-blade impeller creates a characteristic pressure pulse. Should a blade come off, this pulse would change. The sensors in a pressure transmitter would detect this an report and error.
Because level inside a reactor needs to be measured, a DP transmitter with advanced diagnostics is a great strategy for detection process upsets. Moreover, it can be used to detect mixer problems. Operations and maintenance will now know when there is a problem and won’t need to wait until a product is out of specification. And that’s good process reliability!
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